Nanoparticle systems have gained wide attention due to their potential in medicine, such as molecular imaging, immunization, theranostics, and targeted delivery/therapy.1-7 Nanoparticles can be fabricated as strong contrast agents for different imaging modalities with superior signal-to-noise ratios than conventional agents,8 or as therapeutic agents such as drug carriers,9, 10 radioenhancers,11 and photothermal transducers.12 Gold nanoparticles (AuNPs), with their facile synthesis and biocompatibility, have therefore been applied for a variety of therapeutics, especially in cancer therapy.15, 16 
Gold nanostars (NS), with a high absorption-to-scattering ratio in the NIR, efficiently transduce photon energy into heat for hyperthermia therapy.20, 25 To date, most phothermolysis studies utilize laser irradiation higher than the maximal permissible exposure (MPE) of skin by ANSI regulation.26 To make photothermolysis applicable to real practice, one needs to enhance the photothermal transduction efficiency. One way is to use a pulsed laser instead of a continuous-wave laser, permitting efficient photothermal conversion by allowing additional time for electron-phonon relaxation.12, 23, 27 Previously, in vitro photothermolysis using NIR pulsed laser reported irradiances of 1.6-48.6 W/cm2, 23, 28, 29 which were higher than the MPE of skin (e.g. 0.4 W/cm2 at 800 nm). Insufficient intracellular particle delivery and low photothermal transduction efficiency may be the main obstacles. Therefore, there is a strong need to design a more efficient photothermal transducer with optimized cellular uptake.
Recently, star-shaped AuNPs (“nanostars”) have attracted interest because their plasmon can be tuned to the NIR region, and the structure contains multiple sharp tips that can greatly enhance incident electromagnetic fields. Studies have shown that NIR-absorbing nanorods, nanocages or nanoshells can be used as contrast agents in optical imaging techniques such as optical coherent tomography, two-photon luminescence (TPL) microscopy, and photoacoustic imaging. Their large absorption cross-sections can also effectively convert photon energy to heat during photothermal therapy. Nanostars, which absorb in the NIR, have been hypothesized to behave similarly. Nanostar-related bioapplications remain scarce in spite of their potential, mostly due to the difficulty of surface functionalization.
In 2003, Chen et al.53 first reported the synthesis of multipod gold nanoparticles from silver plates in the presence of cetyltrimethylammonium bromide (CTAB) and NaOH. Later, several seedless or seed-mediated synthesis methods were employed using majorly poly(N-vinylpyrolidone) (PVP) or CTAB as surfactant. Further use of nanostars has been limited by (1) the potential toxicity of CTAB, (2) the difficulty of replacing PVP or CTAB during biofunctionalization, and (3) induction of aggregation following multiple washes. Previous experimental studies have shown a red-shifting of the plasmon peak from nanostars with longer or sharper branches. Several numerical studies of their plasmonic properties have recently been reported. Hao et al.'s54 2-D modeling of a single nanostar, consisting of 5 unique branches, with finite difference time domain (FDTD) method showed that nanostars plasmon results from the hybridization of plasmon resonance of each branch; the plasmon peak relative intensity depends on the polarization angle. Sent it et al.55 also stated that the tip angle and radius, but not the number of branches, are the major determining factors in plasmon shift in a simplistic 2-branch model.
To achieve successful and selective photothermolysis or phototherapy, nanostars need to be delivered to the designated target cells without compromising cell viability. This requires overcoming several biological barriers. For example, particles need to be physiologically stable (i.e. non-aggregated, long serum half-life), bind to the cell surface, and traverse the plasma membrane.30, 31 In general, nanoparticle size, shape, surface charge, and coating (e.g. protein corona, polymer, anti-fouling layer) all affect their cellular delivery.32-34 People have tried numerous methods to increase the uptake of nanoparticles. One way to do this is achieved by surface coating with cell penetrating peptides (CPPs).30 
CPPs, with 30 or less amino acids that are cationic or amphipathic in nature, facilitate the translocation across the cellular membrane. Human immunodeficiency virus type 1 (HIV-1) encoded Trans-Activator of Transcription (TAT) peptide, which is one of the most studied CPPs, has been employed to facilitate not only the intracellular delivery of various nanoparticles,35-37 but also the crossing of the blood-brain barrier.38, 39 It has been shown that TAT-labeled proteins and quantum dots (QD) enter cells by lipid raft mediated macropinocytosis,40, 41 which is a particularly enticing uptake pathway in drug delivery because of the large uptake volume, avoidance of lysosomal degradation, and the ease of escaping from macropinosomes due to their inherent leakiness.31 To date, although an enhanced cellular uptake of TAT-labeled gold nanoparticles (TAT-AuNPs) has already been observed,32, 35, 42-46 the cellular uptake mechanism for TAT-AuNP remains unreported.